The technology and principle of optoelectronic distance measurement instrument have developed quickly for more than sixty years since the first patent of an optoelectronic distance measurement instrument was applied in 1947. The optoelectronic distance measurement instrument plays an important role in high-precision measurement, combining with other technology to form multi-function total-station surveying instrument. However, the large-sized surveying instruments are extraordinarily expensive, and the size and weight limit themselves to widely utilized in application field of middle-precision distance measurement for less than 200 meters before 1990s. The low-precision with accuracy up to 2 centimeter ultrasonic ranging devices have been mass produced.
Semiconductor laser diode and digital electronic circuit make great progress until in the late 1990 of the 20th century, and laser become widely used for measuring, phase-shift measuring distance device of millimeter-level measurement accuracy holds a leading post for short-distance measurement for up to 200 meters. Due to the cost effect of handheld short-distance measurement device and the performance difference to the mentioned high-precision long-distance optoelectronic distance instruments, the inventors try their best to research the small-sized short-distance laser distance measurement device.
Laser measuring distance device based on phase shift principle blazes at a tested target by a modulated laser beam, and then a part of the light wave is reflected by the target with corresponding phase shift which is used for distance calculation. However, the measurement accuracy is influenced by the performances of internal electronic components. Higher measurement accuracy requires more precise circuits and relative system design, and importantly the phase drift due to environmental factors on those components should be carefully considered, such as the temperature and the element's life of duty. Generally, well known technique for compensation of the phase shift between internal and external light paths result in elimination of the additional phase drift, and its principle can be illustrated as follows:
Ψin is defined as phase of an internal light path beam, while Ψout is defined as phase of an external light path beam, and ΔΨ is defined as additional phase shift which is generated during the transmission of circuit and photovoltaic conversion. The internal light path beam ein and the external light path beam eout are discriminated respectively by a phase discriminator and have the relation as follows:Φin=Ψin+ΔΨΦout=Ψout+ΔΨ
Wherein ΔΨ is unexpected phase drift depending on different environment, and is unable to be calculated exactly. If the interval switching internal and external light path beams is fast enough, that ΔΨ is thought to the same. Thus the phase difference of the internal and external light path beams is shown as following results:Φ=Φout−Φin=Ψout−Ψin 
Thereby the phase difference Φ has completely eliminated the expected phase which is caused by those environmental and aging factors, in other words, the measurement precision is extremely enhanced.
In prior art, a light beam switching device is used to change light path in order to get internal and external light paths, and electro-mechanical component is widely utilized for this application. However, the response time of this method is as long as hundreds milliseconds, moreover, the large-size device with light beam switching device makes the control circuit complex and leads to higher costs and power consumption. In another prior art, a light wave emitter generates one light wave which is divided into internal and external light path beam by a beam-splitter. Two avalanche photo diodes (APD) become receiver for respectively receiving internal and external light path beams transmitted consequently at the same time. However, two APDs hugely increase system expense.
In another prior art, dual laser diodes (LD) respectively generate internal and external light path beams, and one APD respectively receive the internal and external light path beams to eliminate a fundamental reference. Firstly, the two LDs work in totally different environment and modulation period, which are unable to completely eliminate a fundamental reference; secondly, the discreteness of LD directly results different phase drift even at the same working conditions.
The mentioned three calibration methods in prior art have disadvantages as follows:
(1) Single-emitter and single-receiver system, which means that generating one light path and receiving one light path wherein there is a controlled electro-mechanical component to change internal and external light path beams, and calculating the phases shift of internal and external light wave respectively for phase compensation, thus the unexpected influence of the environment and element aging is eliminated. However, the electro-mechanical switch causes long response time and high costs and power consumption. In the other way, not only the structure of the system is complex, but also it is easy to make mechanical wear causing a short service life, therefore, this method is not widely utilized in industrial application fields.
(2) Single-emitter and dual-receiver system, which means that system generates one light wave and dual receivers respectively receive internal and external light path beams. The phase shifts of internal and external light wave are calculated respectively to get the phase difference, so the unexpected influence of the environment is eliminated. However, this system installs two APDs for respectively receiving internal and external light path beams, thus it cause high system cost.
(3) Dual-emitter and single-receiver system, which means that system generates two light waves and a single-receiver receives these internal and external beams respectively. The phase shift of internal and external light path beams is calculated respectively, thus the unexpected influence of the environment is eliminated. The system installs two light emitters (such as laser diode), and they cause different phase drift due to different temperatures and aging, as a result, phase compensation between two laser diodes cause errors occur during calibrating.
According above discussion, these methods all have limitation in real application.